Recent regulatory developments have led refiners to seek methods for reformulating motor gasolines to meet increasingly stringent air quality requirements. These techniques include reducing the olefin and aromatic content of the motor gasoline while maintaining the desired octane rating by increasing the relative content of isooctane (trimethylpentane) and other octane-enhancing additives such as oxygenates.
Commercial isobutane:butene alkylation, catalyzed by a strong mineral acid such as HF or H.sub.2 SO.sub.4, produces a highly desirable motor gasoline blending component which is enriched in high-octane trimethylpentane. Thus with the advent of more restrictive air quality regulations, the known commercial isobutane:butene alkylation processes present a seemingly ideal solution to the problem of reformulating motor gasoline to minimize both evaporative losses from storage as well as pollutants emissions from gasoline engine operations.
Alkylation is a reaction in which an alkyl group is added to an organic molecule. Thus an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. Industrially, the concept depends on the reaction of a C.sub.2 to C.sub.5 olefin with isobutane in the presence of an acidic catalyst producing a so-called alkylate. This alkylate is a valuable blending component in the manufacture of gasolines due not only to its high octane rating but also to its sensitivity to octane-enhancing additives.
Industrial alkylation processes have historically used large volumes of liquid Bronsted acid catalysts such as hydrofluoric or sulfuric acid under relatively low temperature conditions. Acid strength is preferably maintained at 88 to 94 weight percent by the continuous addition of fresh acid and the continuous withdrawal of spent acid. Liquid acid catalyzed isoparaffin:olefin alkylation processes share inherent drawbacks including environmental and safety concerns, acid consumption, and sludge disposal. For a general discussion of sulfuric acid alkylation, see the series of three articles by L. F. Albright et al., "Alkylation of Isobutane with C.sub.4 Olefins", 27 Ind. Eng. Chem. Res., 381-397, (1988). For a survey of hydrofluoric acid catalyzed alkylation, see 1 Handbook of Petroleum Refining Processes 23-28 (R. A. Meyers, ed., 1986).
Both sulfuric acid and hydrofluoric acid alkylation share inherent drawbacks including environmental and safety concerns, acid consumption, and sludge disposal. Research efforts have been directed to developing alkylation catalysts which are equally as effective as sulfuric or hydrofluoric acids but which avoid many of the problems associated with these two acids, and alternatives such as Lewis acids, e.g., BF.sub.3, have been explored. While Lewis acids generally pose fewer and less severe safety and environmental concerns than strong liquid acids such as HF and H.sub.2 SO.sub.4, it would be desirable to produce paraffin-rich product streams useful as gasoline blending components without the use of noxious and/or corrosive liquid catalyst systems.
The typical petroleum refinery generates numerous olefinic streams, which, upon hydrogenation and optional fractionation, would be useful gasoline blending components. Examples of such streams include the olefinic gasoline and naphtha byproducts of catalytic hydrodewaxing processes such as the MLDW (Mobil Lubricant Dewaxing) and MDDW (Mobil Distillate Dewaxing). Additional examples include olefinic gasoline cuts from delayed coking units (thermally cracked gasoline), as well as from catalytic cracking process units such as a Fluidized Catalytic Cracking (FCC) process. Lighter olefins may be easily dimerized or oligomerized to provide suitable feedstocks, for example in a process such as MOGD/MOGDL (Mobil Olefins to Gasoline and Distillate/ Mobil Olefins to Gasoline, Distillate and Lube Stock), or MOCI (Mobil Olefins to Chemical Intermediates). Examples of processes which product olefinic stocks include the processes taught in U.S. Pat. Nos. 4,922,048 to Harandi and 4,922,051 to Nemet-Mavrodin et al. Additional examples of light olefin dimerization/oligomerization processes include Dimersol (light olefin dimerization), Isopol (selective isobutene isomerization) and Selectopol (selective butadiene polymerization). See Hydrocarbon Processing, Vol. 61, No. 5, May 1982, pp. 110-112, and Hydrocarbon Processing, Vol. 60, No. 9, Sep. 1981, pp. 134-138.
Previously known techniques for hydrogenating olefinic streams required contacting the olefinic stream with molecular hydrogen in the presence of a hydrogenation catalyst at elevated temperature and pressure, for example in the pretreater section of a catalytic reforming process unit. But it is well known that catalytic hydrogenation in the presence of molecular hydrogen poses its own set of safety and environmental concerns, and requires an expensive supply of hydrogen-rich feed gas.
Hydrogenation of selected less highly saturated hydrocarbons by hydrogen transfer from a more highly saturated hydrocarbon has been explored with homogeneous catalyst systems. See for example, D. Baudry, M. Ephritikhine and H. Felkin "The Activation of C--H Bonds in Cyclopentane by Bis(phosphine)rhenium Heptahydrides" J. Chem. Soc. Comm. 1243 (1980), H. Felkin, T. Fillebeen-Khan, R. Holmes-Smith and L. Yingrui "Activation of C--H Bonds in Saturated Hydrocarbons, The Selective Catalytic Functionalisation of Methyl Groups by Means of a Soluble Iridium Polyhydride System" 16 Tetrahedron Letters 1999 (1985), Y. Lin, D. Ma and X. Lu "Iridium Pentahydride Complex Catalyzed Formation of C--C Bond by C--H Bond Activation Followed by Olefin Insertion" 28 Tetrahedron Letters 3249 (1987), R. Crabtree, M. Melles, J. Mihelcic and J. Quirk "Alkane Dehydrogenation by Iridium Complexes" 104 J. Am. Chem. Soc. 107 (1982), D. Baudry, M. Ephritikhine, H. Felkin and J. Zakrewski "The Selective Conversion of n-Pentane into Pent-1-ene via Trihydrido(trans-penta-1,3-diene)bis(triarylphosphine)rhenium" J. Chem. Soc. Comm. 1235 (1982), and M. Baker and L. Field "Reaction of C--H Bonds in Alkanes with Bis(diphosphine) Complexes of Iron" 109 J. Am. Chem. Soc. 2825 (1987). However, these homogeneous catalysts have been found to be unsuitable for industrial application due to several factors, including the inherent difficulties in separating the homogeneous catalyst from the reaction products. Research in the area of hydrogen-deuterium exchange of alkanes over supported metal catalysts has also been disclosed in 108 J. Am. Chem. Soc. 1606 (1986). Alkylation reactions in the presence of solid acid catalysts have also been explored. See A. P. Bolton, Zeolite Chemistry and Catalysis 771 (ACS Monograph 171, 1976).